Cavity resonator



March 29, 1949. w. A. EDsoN 2,465,639

CAVITY RESONATOR Filed Jan. 51, 1945 2 Sheets-Sheet l March 29, 1949. w. A. EDsoN 2,455,639

CAVITY RESONATOR Filed Jan. 31, 1945 2 Sheets-Sheet 2 SILVER SILVER #SILVER m i@ 54 v ALUMINUM /N VEN TOR WA E DSON AT TOR/VEY Patented Mar. 29, 1949 CAVITY RESONATOR William A. Edson, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 31, 1945, Serial No. 575,515

4 Claims.

This invention relates to testing apparatus and more particularly to apparatus for use in ascertaining the effectiveness of object locating systems employing reflected electromagnetic waves. Such testing apparatus sometimes designated as a ring box, echo box, or ringing cavity uses a resonance -chamber which builds up an internal electromagnetic field during receipt of a pulse of oscillations from the transmitter of the object locating system and upon cessation of the pulse returns oscillations of the same frequency to the receiver of the object locating apparatus for a period designated as the "ringtime of the resonance chamber.

An object of the invention is to make more uniform with frequency variation the response characteristics of variable frequency space resonance electrical systems.

Another object of the invention is to provide a testing system of the ringing cavity type in which the ringtime is of substantially the same duration, other conditions being the same, for oscillations of any frequency within a predetermined limited frequency range.

An additional object of the invention is to provide an electrical resonance chamber which may be varied in it-s natural response frequency without greatly changing the time during which it will yield energy stored up in its internal electromagnetic field.

For use in connection with object locating systems it is desired to have the decrement or decay rate of a resonance chamber, commonly expressed in decibels per second, independent of frequency. In other applications of cavity resonators, a selectivity Q, which is independent of frequency, may be desirable. The present invention enables either of these results to be approximated. It also makes it possible to obtain other desired variations of Q or of decrement with reference to frequency.

If a cavity resonator testing apparatus is to be capable of operation in conjunction with all object locator systems operating within some a-ssigned frequency band, it is necessary that the cavity resonator be provided with tuning means. This introduces the problem of securing a uniform response or, in 'other words, a constant ringtime for theca'vity as its tuning is varied from one extreme or limiting frequency to the other.

It has long been the practice in dealing with resonant circuits and electrical systems to appraise them by means of a quality factor designated Q. For electrical resonance chambers operating at microwave frequencies and maintained without load on the cavity at a constant excitation the' unloaded quality factor Q0 is Energy stored If the excitation .of the resonant chamber be applied by a pulse of oscillations of the resonance frequency and of a duration t1, the degree of excitation of the chamber will of course depend upon the peak voltage E measured at the cavity or resonance chamber input and also upon the time t1. The ringtime t which is required for the eld, after cessation of the exciting pulse, to die away to a prescribed minimum voltage Er also measured at the cavity input is, of course, dependent upon the initial energization of the eld of the cavity and upon QL the quality factor of the cavity as loaded by the resistance of the antenna or coupler. By calculations which it is not necessary to pursue the ringtime of a resonant chamber may be shown to be Both theory and practice indicate that when the resonance chamber is tuned irst to oneV of Equation 2. This change in the magnitude of the ringtime is of such nature that beginning at the outermost position of the tuning piston which renders the interior volume of the resonance chamber a maximum and the resonance frequency low will have its greatest magnitude. As the tuning piston moves inwardly to decrease the volume and to increase the natural resonance frequency of the resonance chamber the magnitude of decreases rather rapidly. The rapid change of may not be too troublesome in the case of apparatus in which the tuning range is relatively narrow but it becomes very important where large percentage frequency variations occur as inthe case of test apparatus which must work with various types of transmitters operating at frequencies which may differ rather widely. 'For such wide range tuning apparatus it is highly desirable that the ringtime of the resonance chamber reina-in' reasonably const'ahtmve'r the relatively wide' tuning range.

The explanation of the change in magnitude oi with the change in tuning isftofbefo'undiimthe* fact that QL is a function, ainohgf'otl'eif things;l of the ratio of the diameter D to the'y length IJ of the resonance cavity. of the skin depth, which varies a-s the frequency is varied by changing the ratio D/L. It Happens that these two effects are` compounded so that Q1? decreases vlas iwf increases; with I the? result that thefratio" changes relativelyrapidly'.

It transpires that Qrpisfalsor affunction rof .the resistivity Aoi lthe interior surface. of the resonance chamber and' becomes smaller' with` higher resistivity: Consequently-g. if. it-be possible tain.,-

crease the resistivityv` ofthe interior surface as the volum'e ofy the cavity resonator` is-increased a compensationfor the increaseindnagnitude'ot the ringtime twith. decreasefin -frequency is'vpoa Sl'lblezy pensation for the changei'ringtime 'with change in' tuning, it would be*necess'ary't'ograduate the' resistivity of thel interior wall'surfaces'in a cornplicated manner, whichmi'ght' bev calculated from Equation 2. In practice; however, this is unnecessary since a reasonably satisfactory result may be had and a substantially constant ringtime obtained-by arbitrarily coating. the interior side,

wallsloftheresona'ncechamber''tl'iroughout a portion of .their lengthwith materialhavi'ng av given.

electricity conductivity and' throughout all or portionsv ofthe remainder" with rr'l'aterialv having a different" electrical conductivity.

In the drawing Fig. 1 show's'a diagram, with certain structural features? in partial section, of testing apparatus embodying one form of the invention.

.li-lig.` 2- is a graph indicating the improvement irl-constancy of ringtimewl'lich` is attained; and

Figs. 3,4,15' and--6 illustrate'respectively diil''erent modications of the cavity resonator of' Fig. 1.

Referring to Fig. 1 there is illustrated in section alcavi'ty resonator* It. provided--with.v the' nec.vr essary input and output coupling systems, a detecting and indicating device and a variable tuner. For picking up microwave energy a dipole pi'ckufp" device'- IlA may or may not befproi' vided with a reilectin'g" collector' l Tis shown con'- ne'eteu to a coax-' f cord" IBte'rnnnaung in a" eo axial plug" li associate'dwitlia jack l 5'. Whenitlie' apparatiisis to b'eus'edE as" anon-selective systemV for"dete'rm'iiniiigf the presence-'or'absence of' microwave oscillationsthe' plug F4 i'sremovedfrmtlie It list also a function'` According: teethissinventienthisresult is. attained by makin'gfthat.- portiomothe .side-wallslposition shown and is plugged into jack I6 of the untuned input circuit comprising coaxial conductor" I 'land coupling lOOp;:I 8.5, Thetcoupling loop is Venclosed in 'ashieldingcasing' l 9l which serves also t enclose the rotary coupler and attenua toi-"2th Although for purposes of illustration the easingl I9v is diagrammatically shown in a position considerably separated from the resonator ltfit is to be understood that in actual construction the casing.. l is fitted closely to and supported by the closed end of the cylindrical resonator lil. The details `of this structure are disclosed and claimed in the W. A. Edson Patent 2,414,456, granted' January 21, l9-l, for Electrical testing system. The rotary coupling switch and attenuator Zii'iis mounted? for rotation; by-

erige; shaft aligned 'with .thefaxisof the'c'oanial Becher.- line lcentral conductor 2:1. loop zz'felectricallyf connectedtoasmierowaveides purity siliconirvcrystal.platewith a 4n'e point cone tacti having afcapacity oiithe orderof lamiere.-

microfarads,` the"1 quarter;lr wavelength: coaxial.,

cuit for the unidirectional rectiedi: current e tends from' the 'terminalsffofxthel detectorfbyfway oflexible leadslZElf-to thetermina'lso the milli-f ammeter' 26 and;` the:r` short-ei'rcniting` capacitor;

2l'. The capicitor 21 may have @capacitance-'ot theorder of. limicrofarad and itf'servestozaverage i orfsmooth outrthe` unidirectional currentiten ablethe milliarnmeteriz to give:a.=steadydeec-- the? actual structure it isz preferred; a's--ine the disclosure of the WuAa. EdsorrRatent-.Zgllrli to mi: which reference has' been: madeget'o: incorporate the coaxial Lecher' circuitifZzl'; 25andy 'thefdetector' 23 as" integral parts of.- the` rotating structure'- whicli may accordingly 'beself-shieldingandeeiecsI trically invariablewith :respect to its. connections to the coupler 22.

When'i'n use'to observe-and-:indieate preseence of' microwave f oscillations the coupler: 22

rotated-bymeans ofitssha-fufrom tha position.v shown in'whichf' itfprojects-througihi aperture tif 501V into the'l internalr eld of: the 1 resonator l-- toi a. position inwh'icli it' is adjacent. thecoupling; loop' i8. In` the untuned. input circuit position with the' plug i'4- associated"y Withjacle. l5? microwaveenergy or any frequencywhich the: dipolel l will.

555 pick' up' is transmitted". over tl'l-ecoaxial` circuit'v by Way of couplers It and ZZrto-the-fcircuit olf-.deuw tector 2-3 and theunidir'ectional or-v rectified elec-"- tromotive" forceresulting is applied. over the flex ible cords 2t" to'themilliammeter; 26.

When lit is'VL desir-edftouseA the 1apparaifustoascertain thev frequencyl or;` frequency distribution oi' microwaves the. presence oizwhich hasbeenascertained, the. apparatus: may? be, retinned-` to:v the tuned' input circuit, conditionfshowrriniFig. lwith'; plug.y ldassociated-with:jack:r itfa-ndiwithvthe-rotarycoupling device.'A and 'attenuator in: theposition show-n. in which looprv 22 prcn'eci'sv throughl aper.-` ture Sil. Under' these circumstances, incoming` microwave energyy impressed. upon dipolef I1k is transmitted over the: coaxial cord i3, plug., and jack: M' and l5 respectively, and. the: line: 3.1. to. coupling loop I 32 which. projectsthrough an.` aper-vu ture in the side of the cavity resonator l0 to couple with the'inte'rnal' electromagnetic field of Tt-the resonator;l 'Ehe cavity resonator I0, as is Itfinoludes"v a. coupling;

tector 23 preferablyv oi l at typen-employing. :g aahighzi well known, constitutes a highly selective device which responds strongly to oscillations of a particular frequency or frequencies while discriminating highly against those for which it does not have a natural resonance. Accordingly there will be supplied to coupling loop 22 microwave energy of the selected frequency to which the cavity resonator l is responsive. That selected energy will be detected and indicated in the manner explained in connection with the operation of the untuned input circuit.

For tuning the resonator l0 to any preassigned frequency or for varying the tuning through a range of frequencies in order to measure the frequency of incoming oscillations a reciprocating piston 33 is provided. This may consist of a flat circular disc of brass or aluminum or even of dielectric material provided with an interior surface coating 34 of high conductivity metal such as silver or copper. The position of piston 33 is determined by a plunger or piston rod 35 which fits closely through the circular aperture of the toroidal guide 36 mounted on the cover 31 of the resonator l0. At its outer end the piston rod 35 is connected by a flat spring 38, a coupling rod 39 and a crank fill to a rotating shaft 4I to which is fixed the worm wheel 42 of a worm gear 43. This mechanism is supported on a framework 44 mounted upon the cover plate 31. The worm gear 43 is manually controlled through the tuning knob 45 and shaft 46. Through this mechanism it is possible to move the piston from its innermost position at which it is shown in solid lines to its most outermost position indicated in dotted lines.

The cavity resonator structure I0, as has been suggested, is of circular cylindrical form closed at one end and provided with the cover 31 at the otherwise open end. The resonator I0 may consist of metallic or plastic composition having its inner surface coated with material having high electrical conductivity. In order to achieve stiiness and rigidity of structure with low mass aluminum was used in the particular design illustrated.

The portion of the resonator effective for selectivity at any particular time is that confined between the closed end 48 of the resonator and the coated surface 34 of the piston. The piston ts loosely within the cylindrical structure leaving an annular gap 49 at its periphery to aid in discrimination against unwanted modes of oscillation involving radially directed electric vectors. This annular peripheral gap has very little effect upon oscillations of the desired TEom mode. The aperture 33 is grossly exaggerated in the drawing. In actual practice it comprises a narrow slot extending tangentially with respect to a circle coaxial with the end 48 of the resonator, the circle having a radius of about four-tenths that of the resonator. The slot is made of just sufficient width to enable adequate mechanical clearance of the loop 22 in its motion into and out of the resonator.

It has already been explained that as the tuner piston 33 is moved from its innermost position to its outermost position to increase the effective volume of the resonator, the selectivity factor Q of the resonator increases and its decrement decreases assuming that the interior surfaces of the resonator are of uniform resistivity material. In order to compensate for this effect and to hold the ratio relatively constant with change of tuning the interior surface of the cavity resonator may be coated or plated with a high conductivity coating 50 up to the point 5l which is a small distance inside the innermost position of the piston 33. The coating 50 may, for example, consist of silver or gold. In the design shown it consisted of silver. The thickness of the plating should be several times the skin depth as of the order of 0.0005 inch. Commercial plating is usually sufiiciently thick. The thickness of this plating is so small compared to the diameter of the cavity resonator itself as to have relatively little effect upon the tuning of the resonator. Under these circumstances the coating 34 on the inner surface of the tuning piston may consist of the same material, namely, silver. If the entire inner surface of the resonator l0 were plated with silver, movement of the piston 34 outwardly to increase the enclosed volume would cause increase of the factor Q and of the ringtime of the resonator or decrease of the decrement. In order to prevent this effect that portion of the inner surface of the Walls of the cavity resonator from the point 5l toward the cover 31 is plated as at 52 with a different electrically conducting substance of higher resistivity than the/coating 50. In the particular device illustrated in Fig. 1 the coating 52 is of cadmium. Cadmium was chosen in this instance because it is readily plated and it has a suitable resistivity some four times that of silver. Consequently as the piston 34 is moved outwardly through its tuning range the effective resistivity of its interior surface increases with exposure of the cadmium-coated surface.

In an actual structure designed for operation at wavelengths of 11 to 12.5 centimeters, the internal diameter of the resonator was 6,'7'1 inches, the silver coating extended 3.38 inches from the end of the resonator to the point 5l, the distance to the face of the piston at its innermost position was 3.60 inches and at its outermost position 5.45 inches. This enabled the device to be tuned through a range of frequencies from 2720 to 2890 mc. corresponding to wavelengths extending approximately from 11.0 centimeters to 12.5 centimeters.

The performance of the cavity resonator device of Fig. 1 is illustrated in the graphs of Fig. 2 in which the factor is plotted with reference to the effective length L of the resonator and the relative frequency for oscillations of 'IEo11 mode. It will be apparent that as the frequency extends throughout a range from 1 to 1.12 or approximately 12 per cent change that with an all-silver coating relative v QL varies from 1.4 to unity or, in other words, over a range of about 40 per cent. With the silver and cadmium coating shown in Fig. 1 the range of variation at maximum amounts to only about one-eighth of the maximum for the all-silver coating. It will therefore be apparent that a very marked improvement is obtained and that the actual magnitudes of at the extreme tuning positions differ very little.

Fig. 3 shows a modification of the device of Fig. 1 which provides a less expensive structure.

In this modification the resonance chambe1 54 comprises a brass cylinder having an interior plating 55 of silver extending over the lower end of the cylinder and up the side walls to a point '56 just short of that reached by the inner face of piston 5l which is likewise silver-plated as at 58. When the piston is withdrawn to its uppermost position at 59 the upper unplated portion Eil exhibits the resistivity of brass which is of the order of four times that of silver. This expedient is not particularly satisfactory in the case of resonance chambers constructed of aluminum for the reason that the conductivity of aluminum is not suiciently different from that of silver.

Fig. li discloses a modiiication in which, to approach more nearly theoretically correct graduated plating, two zones of higher resistivity are employed. The lower end of the cylinder and the side walls up to point 6l, which is approximately the lower limit of travel of tuner piston 62, are plated as at 6s with silver as is also the inner face ci the piston 62. The rone over which the piston travels extending approximately from the point el to the point 64 is divided into two regions. The intermediate region from 6l to E55 is plated with a material having a resistivity different from and higher than that of silver. The upper portion from point S5 to 5t is plated with a still different material having a still higher resistivity. This structure permits either a 'wider range of tuning for a given percentage deviation of ringtime or a more constant ringtime for a given range of tuning.

Fig. 5 illustrates the advantage which the invention aiiords in another specific design utilizing TEm mode, covering a frequency band 30% wide. The cylinder l' as in the usual case may consist of aluminum. For one given design the diameter of the cylinder may be eight inches and the silver plating may extend over the interior lower end and also for a distance of four inches up to the point 53 which marks the approximate lower limit of travel of the tuner piston 69. The zone l extends above the point 65 at least as far as the maximum travel of the tuner piston 69. It may be plated with an alloy consisting of 60% copper and li0% nickel. This alloy has a resistivity of about twenty-ve times that of silver. If the structure of Fig. with the dimensions as given were to have its interior surface entirely silver-plated it would exhibit a deviation in ringtime throughout its range of tuning of m35 per cent. However, with the same structure silverplated as before and with the superposed plating of the high resistivity alloy material, the structure exhibits a maximum deviation of ringtime throughout its range of tuning of i9 per cent.

Fig. 6 illustrates an additional improvement which may be attained by a somewhat narrower band of higher resistivity plating. In this structure the internal dimensions of the cylinder and the travel of the tuner piston are the same as in the cases of Figs. 4 and 5. The interior of the cylinder including its lower end surface and the interior surface of the tuner piston 'H are silverplated as at l'Ll. At the point 12 four inches above the lower surface of the cylinder and the approximate lcwer limit of travel of the piston 1I begins a one-half inch wide band 13 throughout which the walls of the cylinder are plated with anV alloy having a resistivity about times that of silver. For this purpose a tin-bismuth alloy comprising 2 per Cent tin and 98 per cent bismuth is suitable. This structure exhibits a maximum deviation in ringtime throughout its range of tuning of 1:5 per cent.

It will be apparent that any desired degree of compensation may be attained by providing a sufficient number of Zones of material of diierent resistivity. It will also be apparent that the resistivity of the surface adjacent those regions in which there is a weak electromagnetic eld for the desired mode of oscillations will have relatively less eiiect but that the resistivity of the surface adjacent the zones of strong electromagnetic held for oscillations of the desired mode Will have a strong effect in determining the ringtime.

It will also be evident that this invention provides a resonant system which may be varied or ,tuned in a continuous manner over a relatively high percentage range of tuning while at the same time automatically varying the damping factor oi the resonant system in such manner as to cause the system to respond for a substantially constant period to an excitation of a given intens-ity.

It will also be evident that other forms of variation of the damping factor with respect to frequency may be achieved, as for example, constant Q with respect to frequency.

What is claimed is:

l. A space resonance chamber having a movable wall to vary its resonant frequency, the interior surface of the iixed Wall being divided into zones of different resistivity in such a way as to produce a desired variation of the decrement with `ing a tuning piston adjacent one end, the interior surface of the cylindrical wall being cadmmm-plated toward the end adjacent the piston and silver-plated toward the opposite end.

3. A space resonance chamber having one wall mounted in the manner of a piston and iitting loosely within the surrounding walls, the surface of the surrounding walls being divided into zones of different resistivity whereby the ringing time of the 'chamber tends to be edualized for diiierent positions of the piston wall.

e. en electrical resonance chamber comprising a cylinder having ene end provided with a piston tuner capable of movement between two positions along the cylindrical wall, the surface of the wall between the positions being cadmiumplated and at least a portion of the remainder of the wall being silver-plated.

WJLLIAM A. EDSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Landon July 8, 1947 

